1. Field of Art
The invention relates to a control device for a hydraulically propelled work vehicle in which both a hydraulic motor for vehicle propulsion and a hydraulic actuator for performing work are driven by a single variable displacement hydraulic pump.
2. Related Art
As a hydraulically propelled work vehicle equipped with this type of control device, there is, for example, a wheel type hydraulic shovel such as that shown in FIG. 9. Referring to that figure, the symbol F denotes a hydraulic motor for vehicle propulsion. Rear wheels M are driven by the rotation of the hydraulic motor F, via a transmission K and a propeller shaft L, so as to propel the vehicle. Further, a boom N, which forms one portion of a front attachment, is raised and lowered by the extension and contraction of a boom cylinder G.
FIG. 10 is a general view of a hydraulic circuit, for vehicle propulsion and for performing work, of a wheel type hydraulic shovel in which load sensing control and input torque limitation control are performed. The reference symbol A denotes a variable displacement hydraulic pump which is driven by an engine B. Its displacement is controlled by a tilt angle control device C in which load sensing control and input torque limitation control are performed. D is a control valve for vehicle propulsion, E is a control valve for performing work, F is a variable displacement hydraulic motor for vehicle propulsion, and G is a hydraulic cylinder for performing work. The hydraulic fluid ejected from the variable displacement hydraulic pump A is supplied, under the respective control of the control valves D and E, to the hydraulic motor F for vehicle propulsion and to the hydraulic cylinder G for performing work. Further, H is a pressure compensation valve which compensates for the fact that each of the actuators F and G is operated by pressures which are mutually independent, I is a counterbalance valve, and J is a center joint which connects together the hydraulic conduits in the upper rotating body portion and in the lower moving body portion.
Load sensing control is a type of control for the displacement of the variable displacement hydraulic pump A (hereinafter also termed the tilt angle) which controls it so as to keep at a constant value the pressure difference between the pressure upstream of the control valve D for vehicle movement or the control valve E for performing work and the pressure downstream thereof, i.e. the pressure difference between the inlet pressure (the pump pressure) to the control valve D or the control valve E and its outlet pressure (which is the greater one of the load pressure of the hydraulic motor F and the load pressure of the hydraulic cylinder G and is called the load sensing pressure). The pump pressure is maintained higher by just a fixed target value than the load sensing pressure.
Input torque limitation control is a type of control for the displacement of the variable displacement hydraulic pump A which controls the pump A so as to keep the pump A to a displacement value q which is calculated from a P-q graph showing the relation between pump pressure P and displacement q, based upon the pump pressure P, and sets the pump A to a displacement such that over the entire engine revolution speed range the engine power is not exceeded, in order to utilize the engine power effectively.
With such a single pump type of wheel type hydraulic shovel, the power required during vehicle propulsion is larger as compared with the power required while performing work, and in the prior art the P-Q graph for the variable displacement hydraulic pump A was set as shown in FIG. 1B. Referring to FIG. 1B, T is the graph applicable to vehicle propulsion, while D is the graph applicable while performing work. To improve fuel economy and control noise, while performing work, the practice is to decrease the engine revolution speed while increasing the maximum displacement of the hydraulic pump. However, the maximum displacement of the hydraulic pump while performing work is set to a value at which a desired front speed, at the maximum engine revolution speed while performing work, can be obtained. Furthermore, under the condition that the pressure during vehicle propulsion and the pressure while performing work are equal, the flow amount during vehicle propulsion is greater than or equal to the flow amount during excavation.
On the other hand, the maximum displacement of the hydraulic motor F is set to a value that permits a vehicle propulsion torque which provides a predetermined hill climbing capability and at which, during vehicle propulsion on level ground, a fixed maximum speed cannot be exceeded. At this time, of course the torque efficiency and the volumetric efficiency of the hydraulic motor should be considered.
As will be understood from FIG. 10, the greater part of the pressure loss in the hydraulic circuit for performing work is related to the control valve E; but in the hydraulic circuit for vehicle propulsion, apart from the pressure loss related to the control valve D, there are also contributions attributable to the counterbalance valve I and to the center joint J. Therefore as a whole, the pressure loss in the hydraulic circuit is quite great as compared to the pressure loss in the hydraulic circuit for performing work. However, as shown in FIG. 1B, in the prior art the maximum pump flow rate Qtmax during vehicle propulsion was arranged to be either identical to or greater than the pump flow rate Qdmax when performing work and the pressure loss during vehicle propulsion, was greater, which left room for improvement from the point of view of efficiency. That is to say, when during vehicle propulsion, in order to increase the power the engine revolution speed was increased and the pump output flow amount was also increased compared to its value while performing work. Then the pressure loss became larger and the propulsion torque became smaller although the motor volume is enough to obtain the desired propulsion torque. Further, the construction of the hydraulic circuit was increased in size, which was an invitation to a cost increase.